In recent years, there have come into widespread use devices, compliant to formats such as MPEG (Moving Picture Experts Group) or the like, which handle image information as digital signals, and take advantage of redundancy peculiar to the image information in order to perform highly effective information transmission and storage at that time, to compress the image by orthogonal transform such as discrete cosine transform or the like and motion compensation, as both information distribution such as broadcasting and information reception in general households.
In particular, MPEG2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2) is defined as a general-purpose image encoding format, and is a standard encompassing both of interlaced scanning images and sequential-scanning images, and standard resolution images and high definition images. For example, MPEG2 has widely been employed now by broad range of applications for professional usage and for consumer usage. By employing the MPEG2 compression format, a code amount (bit rate) of 4 through 8 Mbps is allocated in the event of an interlaced scanning image of standard resolution having 720×480 pixels, for example. Also, by employing the MPEG2 compression format, a code amount (bit rate) of 18 through 22 Mbps is allocated in the event of an interlaced scanning image of high resolution having 1920×1088 pixels, for example, whereby a high compression rate and excellent image quality can be realized.
With MPEG2, high image quality encoding adapted to broadcasting usage is principally taken as an object, but a lower code amount (bit rate) than the code amount of MPEG1, i.e., an encoding format having a higher compression rate is not handled. According to spread of personal digital assistants, it has been expected that needs for such an encoding format will be increased from now on, and in response to this, standardization of the MPEG4 encoding format has been performed. With regard to an image encoding format, the specification thereof was confirmed as international standard as ISO/IEC 14496-2 in December in 1998.
Further, in recent years, standardization of a standard called H.26L (ITU-T (ITU Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Experts Group)) has progressed, originally intended for image encoding for videoconferencing usage. With H.26L, it has been known that as compared to a conventional encoding format such as MPEG2 or MPEG4, though greater computation amount is requested for encoding and decoding thereof, higher encoding efficiency is realized. Also, currently, as part of activity of MPEG4, standardization for also taking advantage of functions not supported by H.26L with this H.26L taken as a base, to realize higher encoding efficiency, has been performed as Joint Model of Enhanced-Compression Video Coding. As a schedule of standardization, H.264 and MPEG-4 Part 10 (AVC (Advanced Video Coding)) become an international standard in March, 2003.
Also, there is adaptive loop filter (ALF (Adaptive Loop Filter)) as a next generation video encoding technique which is being considered as of recent (see NPL 1 for example). According to this adaptive loop filter, optimal filter processing is performed each frame, and block noise which was not completely removed at the deblocking filter, and noise due to quantization, can be reduced.
However, images generally have various features, so optimal filter coefficients are locally different. With the method in NPL 1, the same filter coefficient is applied to all pixels within one frame, so the image quality of the overall frame improves, but there has been the concern that there may be local deterioration.
Accordingly, there has been conceived not performing filter processing in regions which locally deteriorate (see NPL 2 and NPL 3, for example). In this case, the image encoding device corresponds multiple control blocks arrayed without gaps as if they were being used for paving, with regions of the image, and controls whether or not to perform filter processing on the image for each control block. The image encoding device sets flag information for each block, and performs adaptive filter processing according to the flag information. In the same way, the image decoding device also performs adaptive filter processing according to the flag information.